We have all had this happen to varying degrees. Rowdy is right that people just don't know what else to say. They also think that to say they are upset or worried would be upsetting to us. I think that it really depends on who is saying this and our relationship with them. If it is someone we barely know then I think you just let it go. If it is someone close then I think you tell them that it would be more helpful if they listened and tried to understand that sometimes you just need a sympathetic ear.
I am not sure what stage you are at but I also found that some of the chemo drugs would effect my mood for a few days each time. Kind of like really bad PMT! During that time I know that I was probably very irritable and basically nothing anyone said was helpful. My family luckily understood that it was just the drugs and I would come out of it and return to my normal positive self.
Sometimes I would vent on here and many times I found that by the time I had written down my feelings I actually felt ok again. At least we all understand on here. Take care and use this network to find that sympathetic, understanding ear! Deanne xxx